METHYL ALCOHOL 41 25'5 grams of iodine in a solution of 50 grams of potassium iodide in 200 c.c. of water and making the volume up to i litre. (d) Approximately N/io-sodium thiosulphate solution, prepared by dissolving 25 grams of the pure salt to i litre. (e) Fresh starch paste. TITRATION OF THE THIOSULPHATE SOLUTION. To 20 c.c. of an aqueous solution containing 3-863 grams of potassum dichromate per litre are added 10 c.c. of 10% potassium iodide solution and 5 c.c. of hydrochloric acid (D i-io). After mixing, 100-150 c.c. of water are added and the free iodine titrated with the thiosulphate solution. Towards the end of the titration, a little starch paste is added, addition of the thiosulphate being discon- tinued when a drop of the latter changes the greenish blue colour to pale green. Since 20 c.c. of the dichromate solution set free 0-2 gram of iodine from the potassium iodide, the amount of iodine corresponding with i c.c. of the thiosulphate solution may be readily calculated. TITRATION OF THE IODINE SOLUTION. 10 c.c. of the iodine solution are pipetted into a dish and the thiosulphate solution run in from a burette until the liquid becomes pale yellow. Starch paste is then added and the titration continued until the liquid becomes colourless. The amount of iodine in 10 c.c. of the solution is found by multiplying the number of c.c. of thiosulphate used by its iodine equivalent. METHOD OF WORKING. 25 c.c. of the alcoholx are introduced into a litre flask containing about 500 c.c. of water and the solution made up to volume. After mixing, 10 c.c. of the liquid (corresponding with 0-25 c.c. of the methyl alcohol) are placed in a 300-400 c.c. bottle fitted with a ground stopper and containing 10 c.c. of the caustic soda solution (a) ; after mixing, 40 c.c. of solution (c) are added from a burette. The closed bottle is left for an hour with occasional shaking, after which 10 c.c. of solution (b) are added and the non-combined iodine titrated with the sodium thiosulphate solution > (d). Subtraction of non-combined iodine from the amount contained in 40 c.c. of the iodine solution gives the amount reacting.2 If the latter is a grams, the percentage of acetone is given by the formula, 58^05 X a 761-52 X 100. This gives the number of grams of acetone in 100 c.c. of the methyl alcohol; multiplication by 0-7966—the density of acetone at 15°—then yields the percentage of acetone by volume. 7. Bromine Absorption.—From this determination the quantity of various impurities, principally allyl alcohol, is deduced. The reagents required are : (a] A solution of potassium brornate and bromide, prepared thus : Powdered potassium brojnate and bromide (puriss.) are dried separately in porcelain dishes for 2 hours at 100°. 2-447 grams of the bromate and * With rectified methyl alcohol, 50-100 c.c. are taken according to its supposed acetone content. 2 Each rnol. of acetone (58-05) requires 6 atoms of iodine (761-52). the pure